1. Field of the Invention
This invention relates generally to surface modification of inner surfaces of a hydrogen generation reactor chamber, and in particular, to the surface modification of the principal surface of the inner surface of a hydrogen generation reactor chamber using a cold spray of material.
2. Description of Related Art
The growing popularity of portable electronic devices has produced an increased demand for compact and correspondingly portable electrical power sources to energize these devices. Developments in robotics and other emerging technology applications are further increasing the demand for small, independent power sources. At present, storage or rechargeable batteries are typically used to provide independent electrical power sources for portable devices. However, the amount of energy that can be stored in storage or rechargeable batteries is insufficient to meet the need of certain applications.
Fuel cells in general and particularly hydrogen/air fuel cells (H/AFCs) have enormous potential as a replacement for batteries. Because fuel cells operate on very energy-dense fuels, fuel cell-based power supplies offer high energy-to-weight ratios compared with even state-of-the-art batteries. Fuel cells are of particular interest to the military, where significant efforts are being made to reduce the weight of power supplies that soldiers must carry to support high-tech, field-portable equipment. There is also considerable potential for utilizing fuel cell-based power supplies for commercial applications, particularly for portable applications, where small size and low weight are desirable.
A common H/AFC is a polymer electrolyte membrane (PEM) fuel cell. PEM fuel cells are constructed of an anode and a cathode separated by a polymer electrolyte membrane. Functionally, the PEM fuel cells generate electricity by stripping the electrons of hydrogen (forming H+) as the hydrogen from the anode side passes through a PEM membrane, the electrons migrate around the PEM membrane to produce a voltage. The H+ reacts with oxygen at the cathode to produce water.
The Solid Oxide Fuel Cell (SOFC) operates by stripping electrons off oxygen. Negatively charged oxygen ions migrate through an electrolyte membrane and react with hydrogen at the anode to form water.
In both cases, the PEM fuel cell and the SOFC, oxygen can typically be obtained from the ambient atmosphere, only a source of hydrogen must be provided to operate the fuel cell.
Merely providing compressed hydrogen is not always a viable option, because of the substantial volume that even a highly compressed gas occupies. Liquid hydrogen, which occupies less volume, is a cryogenic liquid, and a significant amount of energy is required to achieve the extremely low temperatures required to liquefy gaseous hydrogen. Furthermore, there are safety issues involved with the handling and storage of hydrogen in the compressed gas form or in the liquid form.
One method of producing hydrogen is by processing hydrocarbons such as methane (natural gas), propane, butane, and liquid fuels such as gasoline, diesel and JP-8 or oxygenates such as methanol. The choice of fuel and the choice of the method of processing, such as steam reforming, partial oxidation, and autothermal reforming depends to a large extent on the type of service, such as, portable, stationary or automotive fuel cell power systems. Hydrogen can also be produced by cracking ammonia. The product stream from the fuel processor when a hydrocarbon feed is used contains hydrogen in addition to un-reacted hydrocarbons, other products such as CO, CO2, and diluents such as nitrogen. In essence, the hydrogen concentration in the product stream can be in the 40 to 75 volumetric percent (dry basis) range depending on the type of fuel and the method of processing. Methods such as water gas shift and preferential oxidation are used to reduce the CO concentrations to acceptable levels of no more than 50 parts per million, but increase the complexity of the system.
A challenge in developing low temperature (less than about 650° C.) hydrogen generation systems which reform hydrogen rich fuels such as light hydrocarbons in the C1-C4 range and heavy hydrocarbons such as gasoline, jet fuel and diesel, is that carbon formed as a by-product of mechanisms such as the thermal cracking of the fuels is less prone to be removed by carbon removal mechanisms such as gasification. This problem is particularly severe during the reforming of heavy hydrocarbons. The net result is carbon accumulation in the reactor, commonly known as coking, and has a serious effect on lifetime and reaction efficiencies. The condition is exacerbated if a gasification agent such as hydrogen is removed preferentially during reforming, as is the case when the reformer is operated as a membrane reformer. Operation at low temperatures (<650° C.) therefore necessitates the incorporation of techniques that prevent or minimizes carbon formation in the first place.
Metal surfaces such as those provided by reactor walls and walls of connecting tubing, when untreated, can contain surface oxygen that promote the scission of the C—H bond in hydrocarbons and thereby favors carbon formation and deposition. The deposited carbon, if not removed, serves as nucleation sites for additional deposition of carbon layers. Furthermore, thermodynamics favors the occurrence of carbon formation in the gas phase at conditions that are typically used for hydrogen production. Carbon formation from hydrocarbon raw materials, such as methane, can occur by thermal cracking at temperatures greater than 600° C. and/or from products such as carbon monoxide by the Boudart mechanism, at lower temperatures.
Prior art solutions to reduce coking include steam/carbon feed ratios that are greater than stoichiometric ratios, method of contacting and mixing the hydrocarbon and steam feed, minimizing empty reactor volume, and employing operating temperatures that are greater than 750° C. to favor gasification.
Therefore, it is a desideratum to produce a reactor which reduces carbon formation/coking of the reactor walls and/or connecting tubes.